Variable inductance circuits



May 18, 1954 w. F. SANDS VARIABLE I NDUCTANCE CIRCUITS Filed July 15INVENTOR Wilma EJ123011 ATTORNEY Patented May 18, 1954 VARIABLEINDUCTANCE CIRCUITS William F. Sands, Haddonfield, N. J assignor toRadio Corporation of America, a corporation of Delaware Application July15, 1949, Serial No. 104,984

14 Claims.

This invention relates to signaling circuits which are to be mutuallycoupled and which include inductances of different magnitudes having apredetermined ratio relative to one another, and particularly, to coilstructures providing the inductance in the respective circuits which maybe simultaneously varied by means of a movable core without varyingsubstantially the predetermined inductance ratio of the two circuits.

In signaling systems, such as those included in radio receivers and thelike, there are many cases where two inductive circuits are to bemutually intercoupled and in which the magnitudes of the inductancesincluded in the respective circuits are different and yet have apredetermined ratio with respect to one another and in which theinductance ratio must be maintained substantially constant. Wherecircuits of the character described are to be variably tuned, or atleast are to be controlled in such a manner that correspondingvariations in the respective inductances are to be effected withoutchanging the ratio of the inductances in the respective circuits, it haspreviously been necessary to provide the inductive devices in therespective circuits with individual movable core elements. This has beennecessary when using coil structures of conventional design by reason ofthe fact that in structures of this character a single movable coreelement necessarily affects one inductive device differently from theother.

There are various types of circuits which are to be coupled and variablyadjusted in the manner described wherein it is desirable to effect thenecessary corresponding variations in the inductances included in therespective circuits by means of a single movable core element. One typeof circuit wherein such a facility may be advantageously employed is thecoupling between the output and input circuits of an oscillator. Anothersuch circuit is one in which an impedance transformation is desired. Inoscillators, particularly, which operate by reason of an inductivecoupling between the input and output circuits thereof, it generally isnecessary, in .1

order to effect the proper coupling, that a coil structure be providedin which one winding has a somewhat different inductance than anotherwinding. For example, in the well known type of Hartley oscillator,there is employed a coil cillators operate in a satisfactory manner, itis necessary to maintain a substantially constant ratio between theinductance of the portion of the coil structure included in the anode oroutput circuit of the inductance and that portion of the coil structureincluded in the control grid or input circuit. When using coilstructures of conventional design, it is not possible to employ a singlemovable core element for varying the frequency of the oscillator withouteffecting a greater change in the inductance of one of the coilstructure portions than in the other. As a consequence, the ratio of theinductances in the respective circuits does not remain constant with theresult that the operation of the oscillator is adversely affected.

Similarly, it is frequently desirable to employ an inductive devicewhich is tapped at a point closer to one end than the other in a circuitwherein an impedance transformation is to be made. For example, where arelatively high impedance circuit is to be coupled to a relatively lowimpedance circuit, it is a common expedient to couple a coil structurehaving an inductance of a relatively large magnitude across the highimpedance circuit, one terminal of which is common to a terminal of thelow impedance circuit. By providing a tap on the coil structure at apoint which is closer to that end thereof which is connected to thecommon terminal and by connecting this tap to the other terminal of thelow impedance circuit, only a fractional part of the total inductance ofthe coil structure is effectively included in the low impedance circuit.However, where such a coupling device is to be variably tuned, a movablecore element in conjunction With a conventional coil structure isunsatisfactory for the reason that a movement of the core elementeffects a substantially different change in the inductance of oneportion of the coil structure from that efiected in the other portion,with the result that the ratio of inductance of the two circuits is notmaintained constant.

Accordingly, it is an object of the present invention to provide animproved variable inductance apparatus for coupling two inductivecircuits having respectively different inductances in a predeterminedratio, whereby the respective inductances may be varied without varyingthe inductance ratio.

Another object of the invention is to provide an improved variableinductance coil structure for coupling two inductive circuits havingrespectively different inductances in a predetermined ratio, whereby amovable core element may be variably positioned within the coilstructure to effect a. variation in the inductances of the respectivewindings without substantially changing said inductance ratio.

Another object of the invention is to provide an improved coil structurefor use with an osci1-- lator as the coupling means between the outputand input circuits thereof and which comprises portions having differentinductances in the respective circuits in predetermined ratio, whereby asingle movable core element may be variably positioned within the coilstructure for effecting inductance variations in the respective coilportion in substantially the same proportion.

In accordance with the present invention, there is provided apparatusfor coupling two inductive circuits having respectively differentinductances in a predetermined ratio relative to one another whichcomprises a multi-iilar coil structure having at least three windingsarranged with ad jacent corresponding turns. At least one of thewindings is connected in one of the circuits and a different number ofthe windings is connected in the other of said circuits. A core elementalso is included and is mounted for movement within the coil structureso as to simultaneously vary the inductances of the respective windings,all in substantially the same proportion whereby the ratio of theinductances remains substantially constant.

In accordance with another feature of the invention, there is provided acoil structure substantially of the character described for use incoupling the output and input circuits of an oscillator. At least one ofthe windings is includedin the output circuit of the oscillator and agreater number of windings is included in the input circuit of theoscillator. In a particular form of oscillator all of the windings areconnected in series relationship. In another form of oscillator thewindings associated respectively with the input and output circuits arenot conductively coupled and the output circuit winding is centrallydisposed relative to the input circuit windings.

The novel features that are considered char acteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization andmethod of operation aswell as additional objects and advantages thereof will best beunderstood from the following description taken in connection with theaccompanying drawing.

In the drawing:

Figure 1 is a circuit diagram of that portion of a superheterodyne radioreceiver embodying the present invention wherein a Hartley oscillator isemployed as the local frequency generator in a frequency converterstage, whereby the received radio frequency signals may be changed intointermediate frequency signals;

Figure 2 is a diagrammatic representation of a tri-filar windingtogether with its associated magnetic core element by which it may betuned in accordance with the invention;

Figure 3 is another circuit diagram of a reaction type of oscillatorembodying the invention in conjunction with a triode electronic tube;

Figure 4 is a further embodiment of the invention as it may be used inan impedance transformation circuit; and

Figures 5 and 6 show alternative embodiments of the invention.

Having reference now to Figure 1, there is shown the radiant energyreceiving and frequency converting portion of a superheterodyne radioreceiver. An antenna I9 is coupled by a capacitor H to the signal inputgrid [2 of an electronic tube l3 which as illustrated may be a pentagridtube such as an RCA type 6SA'7. The antenna circuit is resonated by aninductor i l and a shunt-connected capacitor 15. The low potentialterminal of the inductor i4 is bypassed to ground by a capacitor it sothat a connection of this terminal may be made to a conventional AVCcircuit as indicated. Tuning of the antenna circuit is effected byvarying the inductance of the inductor M by means of an associatedmovable core element ll which, it will be understood by those skilled inthe art, preferably is positioned within the convolutions of theinductor and is axially movable 'elative thereto as indicated by thedouble-headed arrow.

The frequency converter tube it also is provided with a cathode 18, anoscillator grid IS, a pair of space charge grids 29 and 2!, a suppressorgrid 22 and an anode Space current and the necessary operatingpotentials for the tube iii are derived from a unidirectional powersupply such as indicated by the battery 24. The two space charge grids2G and 2! are connected together as shown and are supplied with suitablevoltage of positive polarity by means of a connection including aresistor 25 to the positive terminal of the battery 24. The anode 23 ofthe converter tube is coupled through a parallel resonant circuit 23comprising an inductor ill and a resonating capacitor 28 and through aresistor 2f: to the positive terminal of the battery 2 whereby spacecurrent is furnished to the tube l3 and also whereby signal-modulatedintermediate frequency energy is developed in the circuit 2% which istuned for resonance at the intermediate frequency. A capacitor 3%]provides a bypass to ground from the resonant circuit 2% for radiofrequency currents. The suppressor grid 22 of the tube 13 is connecteddirectly to ground as shown.

oscillator rid is of the tube iii is coupled by a capacitor 3i to oneterminal of a multifilar coil structure 32, which is resonated by acapacitor 33. A resistor E i provides a leak to ground for the couplincapacitor 3 l. The multifiiar coil 32 as shown comprises threesubstantially similar windings 35, 3t and 37 connected in series aidingrelationship to ground. The ungrounded terminal of the winding til isconnected directly to the cathode it of the frequency converter tube il. The multi-filar coil it! is provided with a movable core element 38which, as will be more fully described hereinafter, is intima ely andsubstantially equally associated with each of the windings and 3?. Alsothe core 33 is arranged to be axially movable as indicated by thedouble-headed arrow, whereby to effect the desired variation in theinductance of the coil 32 to suitably alter the operating frequency ofthe oscillator for tuning purposes. The tuning cores 6'. and preferablyare arranged. fcr concurrent movement and, accordingly, are suitablylinked mechanically as indicated by the dash line 39.

The intermediate frequency output signal is derived from the frequencyconverted by means of a secondary resonant circuit at comprising theparallel arrangement of an inductor 4! coupled to the primary inductor2"! and a resonating capacitor by means of which the circuit 2-3 istuned for resonance at the intermediate frequency. The terminals of theresonant circuit is may be suitably coupled to an intermediate frequencysignal amplifier as indicated.

Having reference now to Figure 2 of the drawing, there is indicateddiagrammatically the manner in which a multi-filar coil structure may beformed in accordance with. the invention. In the illustrative example,the multi-filar coil comprises three windings which may be supportedupon a coil consisting of a hollow tube 43, preferably of insulatingmaterial. Since there are to be three windings forming the particulartri--filar coil shown for illustrative purposes, there are threeconductors represented herein by the solid line 35, the dash line 35 andthe dot-dash line 3? in order that they may be individuallydistinguished. It may be seen from an inspection of Figure 2 that thethree conductors 35, 36 and 31 are interwound upon the tube 43 in such amanner that each turn or convolution of each winding is adjacent to thecoresponding convolutions of the other windings. The magnetic coreelement 33 is inserted within the hollow tube 3 substantially asindicated. It may be readily seen from this figure that, irrespective ofin what position the core 38 may be placed within the tube substantiallythe same number of turns of each or" the windings 35, 36 and 31 aremagnetically linked thereby. Consequently, whatever change in the valueof the inductance or" one or the windings may be effected by varying theposition of the core 38 within the tube substantially a proportionalchange in the inductance of the other windings will be effected.

Bearin -1 mind the structural arrangement or" the Inuit) l coil and itscooperating tuning core shown in Figure 2, consideration should nowgiven with reference to Figure l of the operation of the embodiment ofthe invention in conjunction with the oscillator stage of a superheterodne radio receiver. As is well known by those skilled in the art, thepentagrid frequency converter tube It includes an oscillator comprisingthe cathode iii, the oscillator grid 59 and effectively the space chargegrids 20 and 21 serving as the oscillator anode or output electrode. Asin a conventional Hartley osci1- later, the cathode i8 is connected tothe feedb ck inductor such as the tri-filar coil 32 at a point locatedapproximately at one-third of the distance from the grounded terminal.Accordingly, a tri-filar coil such as the coil 32 serves effectively inan oscillator of this type by reason of the fact that the cathode is maybe connected to one of the junction points between the windings of thecoil. It readily be seen that, when the magnetic core elements ll and 38are adjusted in position relative to their associated coils i l and tovary the tuning of the receiver, not only is the overall inductance ofthe coil 32 changed in the manner desired but also the inductance oi thewinding i-i'i connected to the oscillator cathode it; is changed insubstantially the same proportion. Therefore, the inter-relationshipbetween the energy developed in the output circuit of the oscillator andthe energy fed back. to the input circuit for producing sustainedoscillations is substantially unaltered when the feedback coil. is atri-filar one in accordance with the invention and the inductancethereof is varied for tuning purposes. The oscillator, as a result,performs with substantially equal facility irrespective of the frequencyat which it is tuned.

Referring now to Figure 3 of the drawing, there is shown another form ofoscillator which may advantageously employ an embodiment of the in- 6vention. In this case, the electronic tube 44 is a triode having adirectly heated or filamentary cathode 65 which may be suitablyenergized by a relatively low voltage unidirectional power supply suchas indicated by the battery it. The anode t! oi the oscillator tube M iscoupled through a centrally disposed winding id of a trifilar coilstructure ill and thence, through a load resistor 56 to the positiveterminal of a source of unidirectional space current such as a battery5!. he resistor Eli and the battery 5i may be bypassed to ground by acapacitor 52.

The grid 5;- of the oscillator tube M is coupled by a capacitor 5% toone terminal of one end winding 55 of the tri-iilar coil structure t9,the other terminal of which is coupled in series aiding relationship toone terminal of the other end windin 55 of the coil structure. The othertenminal of the winding is connected to ground and the two windings and5d are resonated by a capacitor 51. A resistor 58 provides a leakcircult to ground from the grid or" the oscillator tube. The tri-filarcoil structure is provided with a movable core element which, asindicated by the double headed arrow, be axially moved relative to thecoil windings for tuning purposes.

The windings i8, 55 and 56 may be formed upon a suitable hollow tube ina manner substantially similar to that shown in Figure 2. Inasmuch aseach convolution of each of the windings is wound upon the supportingform in Such a manner that it is adjacent to corresponding convolutionsof each of the other windings, the position of the core 59 within thesupporting tube will produce substantially proportional inductancechanges in each of the windings.

The movable core structure 5E3, as used in this form of the invention,may consist of a ferromagnetic portion fill and a substantiallynonmagnetic portion Bi. The non-magnetic portion may be formed bymounting upon an insulating rod 62 a series of finitely spacednon-metallic rings 63 which, for example, may be made or copper. Notonly are the rings 83 finitely spaced with respect to one another, butthere also is provided a finite spacing between the end ring 54 and theadjacent end of the ferromagnetic portion 56. The purpose of such amovable core element is to extend the range over which the tri-filarcoil structure 49 may be tuned.

Briefly, the operation of the oscillator shown in Figure 3 consists ofthe development in the Winding 48 of output circuit energy which isinductively coupled in suitable proportion into the windings 55 and 56so that there is impressed upon the input circuit of the oscillator tubeby means of the capacitor 5 9, energy in suitable magnitude and phase toeffect the generation of sustained oscillations. lI'he magnetic coreelement 59 may be varied in its position relative to the number ofconvolutions oi the windings 48, 55 and 56 which it links so as tosuitably vary the frequency at which the oscillator operates.

During that portion of the movement of the core 59 wherein theferromagnetic portion to only is associated with the windings oi"- thetrifilar coil structure '49, the non-magnetic portion (ii of the corehas no effect upon the coil structure. As the ferromagnetic portion 65is withdrawn from the coil structure in a downward direction, as viewedin the drawing, there is er"- fected a continuous decrease in the valueof the inductances of the respective windings it, 55 and '56,. As thedownward movement of the core 5% is continued the copper rings of thenon-magnetic portion 6| become associated with the windings of the coilstructure it so that the respective inductances of the windings 48, and56 are still further decreased as a result of the production of eddycurrent losses in the spaced non-magnetic rings 63. In other words,maximum inductance of the coil windings is achieved by maximumpenetration of the coil structure by the ferromagnetic portion alone andminimum inductance of the coil windings is achievec when only thenon-magnetic portion ii! is en tirely within structure. Thus, it may bethat the additional decrease in the inductances of the respectivewindings, effected by means of the non-magnetic rings 655 after theferromagnetic portion iiii has been entirely withdrawn from the coilstructure, enables the tuning of the coil structure over a much widerrange than would otherwise be possible.

By virtue of the separation of corresponding turns of the input circuitwindings and of the tri-filar coil structure it, the capacitive couplingbetween the two portions of the input circuit windings is decreasedthereby enabling more eiiicient performance of the oscillator.

Having reference now to Figure l of the draw ing, there is shown anembodiment of the invention for use in effecting an impedancetransformation between two circuits which is desired to couple togetherand which it is to variably tune. The terminals 65 are nected to acircuit having a relatively high inn pedance. There is provided ainulti-fiiar coil structure 655 having in this instance three W ings 6i,E58 and G9. The multi filar winding to is formed substantially in the asat indicated in Figure 2. The individual winare connected in seriesaiding relation shown, for coupling across the terminals and areresonated by capacitor it, also, coupled across the terminals Themulti-iilar coil structure 65 is provided with a movable core element Hby which to correspondingly vary the inductances of all of therespective windi. thereof. The low impedance circuit which is to becoupled to the terminals 65 comprises the cathode circuit of anelectronic tube control grid it of which grounded. cuit connected to thecathode it includes re tively low impedance resistor "iii which passedby a capacitor The output circuit of the tube '12 includes an anode lwhich is sup. lied with space current from a suitable source i cated at+33 through circuit Wi an inductor and a load resistor bypassed toground by a capacitor The c Considering new r m in which multi-filarcoil structure functions to e a satisfactory impedance transformationbetween the relatively, high impedance circuit coupled to the terminalsand the relatively low iinped cathode input circuit of the tubeterminating between the grounded grid '53 and the ungrounded terminal ofthe coil 69, it should be taken into account that these impedanoesreinai tially constant. Also, will be appras the core ii is variablypositioned re veto the windings Bl, es and til, the inductance,therefore the impedance of the rec ings necessarily varies which, ofcourse a variation of the total impedance of the multifiler coilstructure. Consequently, in the present case, a compromise is necessary,whereby the impedances of the windings of the multifilar coil structureare chosen so that, at a pre determined intermediate point of the rangeof adjustment of the core 'H, a substantially exact impedancetransformation is efiected. For example, the center point of the range,in most cases, will produce optimum performance. Obviously, for allother points of the range the impedance transformation will not beexact, but ordinarily is satisfactory. However, for all points in therange of adiusment of the core H, the impedance ratio of the twoportions of the multi-filar coil structure Sit which are coupledrespectively to the high and low impedance circuits remainssubstantially constant and substantially equal to the impedance ratio ofthe circuits. Accordingly, in this description and in the appendedclaims, it will be understood that the values given are those pertainingto the previously described intermediate point of the range ofadjustment of the core H.

The total impedance of the serially connected windings fil, 53 and @9 issubstantially equal to the impedance of the circuit connected to theterminals 65. The impedance of the winding 59 is substantially equal tothe impedance or" the cathode input circuit of the tube '32. In theembodiment of the invention illustrated in Figure 4i, it is assumed thatthe impedance of the circuit connected to the terminals 65 issubstantially nine times the impedance of the cathode input circuit ofthe tube '12. Since all of the windings 68 and E9 of the multi-fi1arcoil structure 66 are substantially identical and are substantiallyunity-coupled by virtue of the described nianner in which the coilstructure 68 made, it is seen that the total impedance of the seriallyconnected windings is substantially nine times the individual impedanceof any one of the windings such as the winding Obviously, if the ratioof the relatively high pedance circuit to the relatively low impedancecircuit is greater or less than nine, a difierent number of windingsand/or different circuit connections thereto may he made within thescope of the present invention.

For example, in the event that the impedance of the circuit connected tothe terminals bears a relationship to the cathode input impedance of thetube '52 such as substantially 9%, three windings may be employed in theniulti-illar coil structure 86 connected substantially shown in Figure 5across the terminals 65. In this case, it will be noted that thecathode-connected re sister 55 is connected to the junction pointbetween windings i5! and B8.

In the event that the impedance of the circuit connected to theterminals 55 is substantially sixteen times the impedance of the cathodeinput circuit of the tube i2, Figure 6 illustrates an embodiment of theinvention for coupling two such circuits. In this case, a multhfilarcoil structure 32 is provided consisting of four substantially identicalwindings 83, 8t, 35 and 86, all of which are wound upon a supportingform in a manner similar to that shown Fig re 2. The windings are conected in series aiding relationship, the entire series arrangement beingconnected across the relatively high impedance terminals 65 The cathoderesistor (not shown) is connected to the junction point between windingsand 86. It may be seen, therefore, that by reason of the substantialidentity of the individual windings, the 16:1 impedance ratio may 9 bereadily effected by coil structure substantially as shown in Figure 6.

It is to be understood that coil structures in accordance with thepresent invention are not necessarily limited for use as impedancetransformation devices wherein a relatively high impedance outputcircuit is to be coupled to a relatively low impedance input circuit asshown in Figures 4, and 6. Alternatively, it obviously is within thescope of this invention to employ coil structures of the characterdescribed herein for effecting impedance transformations betweenrelatively low impedance output circuit a relatively high impedanceinput circuit. In

order to effect this type of impedance trans formation, it merely isnecessary to reverse the connections of the coil terminals to theirrespective circuits. For example, in either of Figures 4, 5 and 6, theterminals 65 would be connected to the relatively high impedance inputcircuit, while the terminals shown in these figures as connected to theinput circuit of an electronic tube would be coupled instead to theoutput circuit of an electronic tube, for example, the impedance ofwhich would be less than the impedance of the input circuit of thefollowing stage.

It should be clear from a consideration of the several illustrativeembodiments of the invention disclosed herein, that the inductance orimpedance ratios between the two circuits which are to be coupled bymeans of the improved coil structures determines the minimum number ofindividual windings comprising the coil structure, and also therespective number of windings included in the two circuits. For example,where the impedance ratio is 9:1 or its reciprocal 1:9, three windingsor any multiple thereof are required in the coil structure as shown inFigure 4. In such a case, two windings or a multiple thereof areincluded in one circuit and one winding or a multiple thereof isincluded in the other circuit. Also, where the impedance ratio is 9:4 orthe reciprocal thereof, the coil structure may comprise three windingsor a multiple thereof connected as shown in Figure 5. Also, as onefurther example where the impedance ratio is 16:1, 16:9 or thereciprocals thereof, four windings or a multiple thereof are required inthe coil structure. Typical circuit connections for a 16:1 impedanceratio are indicated in Figure 6. Further extensions of the principlesunderlying the present invention are considered to be obvious in view ofthe foregoing illustrative examples whereby coil structures inaccordance with this invention may be provided for coupling two circuitshaving substantially any inductance ratio other than unity.

It will be seen from the foregoing disclosure of several illustrativeembodiments of the invention that there is provided an improved coilstructure for coupling substantially any two inductive circuits havingdifierent inductances in predetermined ratios relative to one another,both or which inductances are to be correspondingly varied by means of acore element movable relative thereto without substantially changing theinductance ratio. Consequently, coil struc tures of the type embodyingthis invention make it possible to simultaneously vary the inductance bya movable core means of numerous circuits which heretofore have requiredindividual sepa rately movable core means.

It, therefore, will be apparent to those skilled in the art that theinvention may be embodied in various forms other than those specificallydisclosed herein for illustrative purposes. Accordingly, the scope ofthe invention is to be determined particularly by reference to theappended claims.

What is claimed is:

1. Apparatus for coupling two inductive circuits having differentinductances in a predetermined ratio relative to one another, saidapparatus comprising a multi-filar coil structure having at least threewindings arranged with adjacent corresponding turns, at least one ofsaid windings being connected in one of said circuits, at dine-r ntnumber of said windings being included in the other of said circuits,and a core element adjustably positionable within said coil structure,whereby to simultaneously vary the inductances of said respectivewindings all in substantially the same proportion so as tocorrespondingly vary the inductances in said circuits while maintainingsaid predetermined ratio substantially constant.

2. Apparatus for coupling two inductive circuits as defined in claim 1,wherein said multifilar coil structure comprises three windings, two ofwhich are included in one of said circuits and the third of which isincluded in the other of said circuits.

3. Apparatus for coupling two inductive circuits as defined in claim 1,wherein all of said windings are interconnected in series aidingrelationship.

4. Apparatus for coupling two inductive circuits as defined in claim 3,wherein a tap is provided at the junction point between two of saidwindings so as to locate said tap at a point which is unsymmetricalrelative to the series connection of said windings.

5. Apparatus for coupling two inductive circuits as defined in claim 1,wherein said multifilar coil structure comprises an odd number ofwindings.

6. Apparatus for coupling two inductive circuits as defined in claim 5,wherein a centrally disposed one of said windings is included in one ofsaid circuits and the remainder of said windings are connected in seriesaiding relationship and are included in the other of said circuits.

7. Apparatus for coupling two inductive circuits as defined in claim 6,wherein said multifilar coil structure comprises three windings.

e. an osciliator including an electronic tube nput circuit electrode, anoutput circuit electrode and a third electrode common to both input andoutput circuits, a circuit coupling input and output circuits,comprising a multifilar coil structure having at least three mutuallyclosely coupled windings in which each of the turns of each winding isadjacent to corresponding turns cf each of the other windings, circuitmeans connecting at least one of windings to output circuit electrodeand a different number of said windings to said input circuit electrode,capacitance coupled to said coil structure for resonating certain onesof said windings at oscillator frequency deterrc ned by the values ofsaid capacitance and the inductances oi respective windings, and a coreelement adjustably positionable within said c s" cture to vary theinductances oi all of dings substantially in the ea .c proportion,whereby to vary said oscillator f1 ,uency.

9. An oscillator as defined in claim 8, in which at least two of saidwindings are connected to one another in series aiding relationship.

10. An oscillator as defined in claim 9, in which two of saidinterconnected windings are coupled between said input circuit electrodeand said common electrode.

11. An oscillator as defined in claim 10, in which a third one of saidwindings is coupled between said output circuit electrode and saidcommon electrode.

12. An oscillator as defined in claim 8, in which at least one ofwindings is coupled in one circuit between said output circuit and saidcommon electrodes, and the others of said windings are connected inseries aiding relationship and are coupled in another circuitmagnetically as sociated only with said one circuit between said inputcircuit and said common electrodes.

13. An oscillator as defined in claim 12, in which said multi filar coilstructure comprises three windings of which two are connected in series.

14. An oscillator as defined in claim 8, in which said multi-iilar coilstructure comprises three windings of which the central one is coupledbetween said output circuit and said com- 12 mon electrodes, and the twoend ones of said windings are connected in series. aiding relationshipand are coupled between said input circuit and said common electrodes.

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